This disclosure relates to photonic crystals and methods for fabricating photonic crystals and, in particular, is related in one aspect of the invention, to a photonic bandgap structure and method using a relatively high-index of refraction (“n”) material having application in optical interconnection of semiconductor integrated circuits, for example. Related applications of this technology may be found in laser cavities, waveguides, high-Q micro cavities, Bragg reflector, super-prism self collimation, photonic crystal fiber, channel drop filter, optical interconnects, and hetero-structure beam splitters, for example.
A photonic crystal is a periodic arrangement of material structures, dielectric or metallic, of arbitrary shape, circles, squares, spheres, cubes, etc. This field has experienced tremendous growth, attributed to several factors, including increasing computational power available to researchers to study the interaction of light with structured matter, the wealth of phenomena discovered computationally and confirmed experimentally in both microwave and optical regimes, and development of fabrication methods capable of controlled structuring of material at scales commensurate with near infrared and visible wavelengths. Perhaps more importantly, the transition of the telecommunication industry to optical fibers for transporting voice and data, and the growing interest in developing low cost components for all-optical networks, has made the replacement of electrical wiring of an integrated circuit with optical structures technically feasible for more efficient photonic interconnection and on-chip communication.
The mechanism of operation of photonic crystals relies on the wave nature of light. Because light is a wave, when it encounters a periodic structure such that its period is comparable to the wavelength, coherent scattering will occur. Indeed, a similar situation is found in the study of solid state physics, where the wave function of an electron is modulated by the periodic potential of the crystalline lattice. This coherent scattering modifies the wave function of the electron which, in some cases, results in the opening of an electronic bandgap.
The presence of an electronic bandgap has profound consequences in electrical and thermodynamic properties of solids. Similarly, when periodicity is introduced in the dielectric constant “∈” (where ε=n2, for non-magnetic materials) of a medium, a photonic bandgap can open. When this happens, electromagnetic waves of certain frequencies, e.g., those that fall within the bandgap, are forbidden to propagate through a medium having such a structure. This is the premise on which suppression of spontaneous emission has been achieved. Indeed, if light of a certain frequency, or equivalently, a photon of certain energy, is forbidden to propagate through the medium surrounding an atom, this atom will not be able to radiate photons of this energy. Thus, certain direct transitions between energy levels will be forbidden, and spontaneous emission will be suppressed.
For a complete photonic bandgap to exist, a fully three-dimensional periodic structure is required. From a practical perspective, it is useful to distinguish between two-dimensional (2D) and three-dimensional (3D) photonic crystals. A 3D photonic crystal consists of periodic arrangement of material structures such that the periodicity occurs in all three spatial dimensions. An example of 3D photonic crystal structure 100A is illustrated in FIG. 1A.
In contrast, in a 2D photonic crystal, the periodicity occurs only in two spatial dimensions. FIG. 1B shows an example of 2D photonic crystal 100B in the form of a perforated slab of dielectric. Clearly, in order to confine light in all dimensions, 2D photonic crystal 100B requires a different mechanism for the confinement of light in the third dimension. In the case of the structure presented in FIG. 1B, total internal reflection is often used.
While the lack of a full three-dimensional photonic bandgap in the case of 2D photonic crystal 100B is certainly a disadvantage of this configuration, the structure has important advantages over 3D photonic crystal 100A. First, the analysis of 3D photonic crystals requires full 3D simulations. Such simulations are computationally expensive, and thus allows for the analysis of only a limited number of geometries. On the other hand, even though a perforated slab is, in fact, a three-dimensional structure, 2D simulations often provide relatively good approximation of the interaction of light with the 2D structure. Furthermore, fabrication of 3D structures at scales required for a photonic bandgap to open at useful or desired optical wavelengths is difficult, given current technological constraints.
In further contrast, planar technologies developed for fabrication of semiconductor integrated circuits are well-suited for the fabrication of structures with features comparable to the wavelength of light, and thus provide a possible opportunity for structuring 2D photonic crystals.
Although methods based on semiconductor technology have been developed for fabrication of 2D photonic crystals, there is still room for improvement in terms of cost reduction, feasibility, and scalability to high volume production.
Thus, what is needed is a practical, less expensive fabrication method for producing 2D photonic crystal structures which is suitable for high-volume production, as compared to the traditional way of patterning planar photonic crystals in semiconductors.